This invention relates to a polishing film for use in polishing the surface of materials such as metals, ceramics, plastics and glass as well as to a method of producing such a film. More particularly, this invention relates to a polishing film for the surface finishing of precision instruments such as optical fiber connectors for communications, color filters for liquid crystal displays, optical lenses, magnetic disk substrates and semiconductor wafers that require a very high degree of surface smoothness, as well as a method of producing such a polishing film.
If there are unwanted scratch marks or protrusions on the surface of such an instrument requiring a high degree of smoothness, the instrument may cease to be able to function as expected according to its design. Thus, the final polishing is a very important step that controls the functions of such a precision instrument. For example, optical fibers for communications are coming to be commonly used as communication lines instead of the traditional copper wires, and they are usually connected by means of connectors. Such optical fiber connectors are formed by passing one or more optical fibers through the center of a ferrule made of partially stabilized zirconia obtained by adding aluminum to yttria and adhesively attached to the ferrule by means of a resin adhesive but their end surface must undergo a final surface polishing process such that the end parts of the optical fibers will not protrude by more than 0.1 μm or indent by more than 0.05 μm from the end surface of the connector. If there is left at the end part of the optical fibers a damage or a left-over adhesive spilled out of the gap with the ferrule, or if the end part of the optical fibers is polished excessively and becomes deformed, furthermore, scattering of light and other phenomena take place at the end part of the optical fibers and the transmission characteristics of the communication system as a whole come to frustrate the original design. Thus, the end part of such optical fibers must be smoothed to a high level of precision.
For the finishing of the end part of optical fibers and the polishing of surfaces of other precision instruments, it has been known to use a polishing tape having a polishing layer of abrading particles (very small spherical silica particles with average diameter of 0.001-0.5 μm) and a resin binder formed on the surface of a plastic film either in the form of a tape or by cutting into another shape, as disclosed, for example, in Japanese Patent Publications Tokkai 11-333731 and 11-333732.
The surface smoothness of a precision-requiring component can be improved by using such abrading particles with very small diameters so as to increase the contact surface of individual particles that act on the surface to be polished. Since the polishing layer is extremely flat, however, there arises the problem of lowered polishing capability such that the throughput is adversely affected and locally unpolished spots are left.
The polishing power can be increased while using abrading particles with such very small diameters if the content of silica particles inside the polishing layer is increased such that the number of abrading particles per unit area contacting the target surface to be polished is increased. This, however, causes the contact area of the resin binder inside the polishing layer to be reduced, and this tends to cause the polishing layer to peel off locally during a polishing operation. Such peeled-off portions of the polishing layer tend to damage the surface of the precision instrument, frustrating the effort to carry out a precise surface finishing process.
The polishing power can be increased also if relatively larger abrading particles are used. If the diameters of the abrading particles are increased, however, the contact area of the resin binder in the polishing layer with the plastic film increases and hence the problem of the polishing layer locally peeling off the plastic film can be alleviated but there arises instead the problem that the smoothness of the polished surface of the precision instrument is adversely affected.
As explained above, the smoothness of the polished surface of a precision instrument, the polishing power and the phenomenon of the polishing layer peeling off the plastic film are all dependent heavily on the size of the abrading particles. Thus, the selection of the size of abrading particles is an important problem in the technical field of surface finishing of a precision instrument.